https://docs.macchina.cc/projects/mini-project-tutorials/a0-can-point-to-point

A0_P2P.ino

#include "esp32_can.h"

void setup()
{
  Serial.begin(115200);
  
  CAN0.setCANPins((gpio_num_t)4,(gpio_num_t)5);
  
  Serial.println("Initializing ...");
  
  // Initialize CAN controller at the specified speed
  if(CAN0.begin(500000))
  {
    Serial.println("Init OK ...");
  } 
  else
  {
    Serial.println("Init Failed ...");
  }
  
  CAN0.watchFor(); //allow anything through
  
  Serial.println("Ready ...!");
  CAN_FRAME txFrame;
  txFrame.rtr = 0;
  txFrame.id = 0x123;
  txFrame.extended = false;
  txFrame.length = 4;
  txFrame.data.byte[0] = 0x10;
  txFrame.data.byte[1] = 0x1A;
  txFrame.data.byte[2] = 0xFF;
  txFrame.data.byte[3] = 0x5D;
  CAN0.sendFrame(txFrame);
}

byte i=0;

// CAN message frame
CAN_FRAME message;

void loop()
{
  if (CAN0.read(message))
  {
    // Print message
    Serial.print("ID: ");
    Serial.println(message.id,HEX);
    Serial.print("Extended: ");
    if(message.extended)
    {
      Serial.println("Yes");
    }
    else
    {
      Serial.println("No");
    }
    Serial.print("Length: ");
    Serial.println(message.length,DEC);
    for(i = 0;i < message.length; i++)
    {
      Serial.print(message.data.byte[i],HEX);
      Serial.print(" ");
    }
    Serial.println();
    Serial.println();
    
    // Send out a return message for each one received
    // Simply increment message id and data bytes to show proper transmission
    // Note: this will double the traffic on the network (provided it passes the filter above)
    message.id++;
    for(i = 0;i < message.length; i++)
    {
      message.data.byte[i]++;
    }
    CAN0.sendFrame(message);
  }
}